The present invention relates to a semiconductor radioactive ray detector having a semiconductor radioactive ray detecting element, a radioactive ray detection module, and a nuclear medicine diagnosis apparatus using the radioactive ray detection module.
Equipment based on the radioactive ray measuring technique has been becoming more and more widespread. In particular, the tendency is remarkable in the field of nuclear medicine, and the representative equipment include positron emission tomographic equipment (PET equipment), single photon emission tomographic equipment (SPECT equipment), and a gamma camera. A radioactive ray detector generally used in the equipment is a combination of a scintillater and a photoelectric multiplier. A scintillater emits light when radioactive rays enter, and the weak light is amplified by the photoelectric multiplier for detection of radioactive rays. Instead of the scintillater, a semiconductor radioactive ray detector using a semiconductor made of a compound such as cadmium telluride (described as CdTe) may be used for measuring radioactive rays. In a semiconductor, when radioactive rays enter, an electric charge, in the form of holes or electrons, is generated because of the photo-electric effect, and the holes or electrons move in an electric field generated by an external voltage applied to the semiconductor. Since the quantity of electric charge is in proportion to the energy of radioactive rays, the energy of the radioactive rays can accurately be detected by accurately measuring the quantity of electric charge.
The CdTe described above has a high effective atomic number as a material for a semiconductor and the sensitivity is high. Furthermore the CdTe has a large band gap of 1.4 V and can work at the room temperature, but also the scintillater has a large atomic number, and there is the need that the sensitivity of CdTe should be made higher to cope with the large atomic number of the scintillater. To satisfy this need, it is conceivable to make a volume of the CdTe portion larger. When a volume of the CdTe portion is made larger, the performance, such as the energy resolution, may disadvantageously decrease. This phenomenon occurs because a carrier life and a carrier mobility of CdTe are not sufficiently long nor large, and also because, when the volume is larger, the carriers easily recombine along the way. When the percentage of recombining and disappearing of carriers is high, a quantity of electric charge cannot accurately be measured. Furthermore, when the volume is larger, a period of time required for movement of carriers becomes longer, so that the electric charge moves for a long time, and the time precision for identifying a point of time when gamma rays enter is deteriorated. This is not advantageous especially for simultaneous measurement of disappearing gamma rays like that performed in the PET equipment. Namely, when the time precision is low, gamma rays cannot be identified discretely, and the direction in which the gamma rays enter cannot be determined disadvantageously.
To overcome the problem described above, it is conceivable to laminate thin crystal of CdTe for use. With this method, because the crystal is thin, a period of time it takes for movement of carriers is short, and the laminated structure allows increase in the volume. The structure can be realized by alternately laminating substrates and crystals as disclosed in Japanese Patent Laid-Open No. 7-50428. Furthermore, when the electrode plates and crystals are alternately laminated as the disclosed in Japanese Patent Laid-Open No. 11-281747, the crystals can be positioned more closer to each other, which enables improvement of the sensitivity.
However, when the electrode plate and CdTe were actually adhered to each other with a conductive adhesive agent and laminated alternately to produce the semiconductor radioactive ray detector, the performance of the semiconductor radioactive ray detector, namely the energy resolution and the time precision thereof were substantially inferior to those of the semiconductor radioactive ray detector without a layered structure. In this case, a number of the semiconductor radioactive ray detectors were not actually used in the measurement.
It was expected that the phenomenon occurs because of deterioration of CdTe caused by a thermal stress. To solve the problem, the conductive adhesive agent was exchanged with a softer conductive agent that more easily followed the thermal stress to produce the semiconductor radioactive ray detector. The thermal stress was sufficiently alleviated, and the number of significantly defective products unusable for measurement was substantially reduced.
However, the energy resolution or the time precision of the semiconductor radioactive ray detector were contrary to the present inventors' expectation and could not reach a sufficiently satisfactory level.